Pseudo-classes and pseudo-elements

What are Pseudo-classes?

A pseudo-class is used to define a special state of an element. For example, it can be used to:

Style an element when a user mouses over it
Style visited and unvisited links differently
Style an element when it gets focus

Syntax

selector:pseudo-class {
    property: value;
}

A common example: anchor Pseudo-classes

Unvisited: a:link
Visited: a:visited
Mouse Over: a:hover
Selected: a:active

All CSS Pseudo Classes

Selector	Example	Description
:active	a:active	Selects the active link
:checked	input:checked	Selects every checked `<input>` element
:disabled	input:disabled	Selects every disabled `<input>` element
:empty	p:empty	Selects every `<p>` element that has no children
:enabled	input:enabled	Selects every enabled `<input>` element
:first-child	p:first-child	Selects every `<p>` elements that is the first child of its parent
:first-of-type	p:first-of-type	Selects every `<p>` element that is the first `<p>` element of its parent
:focus	input:focus	Selects the `<input>` element that has focus
:hover	a:hover	Selects links on mouse over
:in-range	input:in-range	Selects `<input>` elements with a value within a specified range
:invalid	input:invalid	Selects all `<input>` elements with an invalid value
:lang(language)	p:lang(it)	Selects every `<p>` element with a lang attribute value starting with “it”
:last-child	p:last-child	Selects every `<p>` elements that is the last child of its parent
:last-of-type	p:last-of-type	Selects every `<p>` element that is the last `<p>` element of its parent
:link	a:link	Selects all unvisited links
:not(selector)	:not(p)	Selects every element that is not a `<p>` element
:nth-child(n)	p:nth-child(2)	Selects every `<p>` element that is the second child of its parent
:nth-last-child(n)	p:nth-last-child(2)	Selects every `<p>` element that is the second child of its parent, counting from the last child
:nth-last-of-type(n)	p:nth-last-of-type(2)	Selects every `<p>` element that is the second `<p>` element of its parent, counting from the last child
:nth-of-type(n)	p:nth-of-type(2)	Selects every `<p>` element that is the second `<p>` element of its parent
:only-of-type	p:only-of-type	Selects every `<p>` element that is the only `<p>` element of its parent
:only-child	p:only-child	Selects every `<p>` element that is the only child of its parent
:optional	input:optional	Selects `<input>` elements with no “required” attribute
:out-of-range	input:out-of-range	Selects `<input>` elements with a value outside a specified range
:read-only	input:read-only	Selects `<input>` elements with a “readonly” attribute specified
:read-write	input:read-write	Selects `<input>` elements with no “readonly” attribute
:required	input:required	Selects `<input>` elements with a “required” attribute specified
:root	root	Selects the document’s root element
:target	#news:target	Selects the current active #news element (clicked on a URL containing that anchor name)
:valid	input:valid	Selects all `<input>` elements with a valid value
:visited	a:visited	Selects all visited links

What are Pseudo-Elements?

A CSS pseudo-element is used to style specified parts of an element. For example, it can be used to:

Style the first letter, or line, of an element
Insert content before, or after, the content of an element

Notice the double colon notation - ::first-line versus :first-line

The double colon replaced the single-colon notation for pseudo-elements in CSS3. This was an attempt from W3C to distinguish between pseudo-classes and pseudo-elements. The single-colon syntax was used for both pseudo-classes and pseudo-elements in CSS2 and CSS1.

CSS - The ::after Pseudo-element

The ::after pseudo-element can be used to insert some content after the content of an element. The ::before pseudo-element insert the content before the element. Obviously.